Postgres user does not exist?

I have just installed Postgres and have been tinkering with it and various configurations for 1-2 hours.

I am stuck on being unable to change to the postgres-user

$ su - postgres yields the following error: su: unknown login: postgres

$ sudo -u postgres psql yields the following error: sudo: unknown user: postgres

These attempts are made as the normal user. Trying them as root has the exact same results. I have installed postgres via Homebrew on OS X, and I have read the instructions multple times.

psql: Logs me in with my default username

psql -U postgres: Logs me in as the postgres user

Sudo doesn't seem to be required for me.

I use Postgres.app for my OS X postgres database. It removed the headache of making sure the installation was working and the database server was launched properly. Check it out here: http://postgresapp.com

Edit: Credit to @Erwin Brandstetter for correcting my use of the arguments.

By psql --help, when you didn't set options for database name (without -d option) it would be your username, if you didn't do -U, the database username would be your username too, etc.

But by initdb (to create the first database) command it doesn't have your username as any database name. It has a database named postgres. The first database is always created by the initdb command when the data storage area is initialized. This database is called postgres.

So if you don't have another database named your username, you need to do psql -d postgres for psql command to work. And it seems it gives -d option by default, psql postgres also works.

If you have created another database names the same to your username, (it should be done with createdb) then you may command psql only. And it seems the first database user name sets as your machine username by brew.

psql -d <first database name> -U <first database user name>

or,

psql -d postgres -U <your machine username>
psql -d postgres

would work by default.